. A manual of zoology. Zoology. 120 GENERAL PRINCIPLES OF Genital Ducts.—Very frequently in tlie animal kingdom the excretory apparatus furnishes outlets for the sexual products. In the annelids a)id in the vertebrates portions of the nephridial system, either exclusively or in addition to their excretory function, become accessory sexual organs. Hence we sjoeak of a vrogcnital system. This remark- able connexion of genital and excretory organs has a double cause, a physiological and an anatomical. Physiologically important is the fact that eggs and spermatozoa behave like excreta; b
. A manual of zoology. Zoology. 120 GENERAL PRINCIPLES OF Genital Ducts.—Very frequently in tlie animal kingdom the excretory apparatus furnishes outlets for the sexual products. In the annelids a)id in the vertebrates portions of the nephridial system, either exclusively or in addition to their excretory function, become accessory sexual organs. Hence we sjoeak of a vrogcnital system. This remark- able connexion of genital and excretory organs has a double cause, a physiological and an anatomical. Physiologically important is the fact that eggs and spermatozoa behave like excreta; being substances which are no longer destined for the use of the individual, but must reach the exterior in order to become efflcient. The morphological cause is the relation to the ccelom. A urogenital system is formed only in animals in which the germinal epithelium arises from the epithe- lium of the ctelom, and in which the kidneys or their rudiments stand permanenth' in connexion with the body cavity and thus ,4,^S'^sfmtaaus!'p: form the natural outlet'for its products. "'tt°uterus^'*? Whether the accessory sexual parts are por- organs or are inde- pendent structures, they have in the animal series a definite arrangement adapted to their function (fig. 73). Canals lead from the sexual glands to the exterior, the oviducts in the female, the vasa deferentia in the male (and the herma- phroditic duct from the hermaphroditic gland). Accessory Sexual Apparatus.—The terminal portion of the vas deferens is often very muscular and is ciilled the ductus ejacvla- torius; it may be evaginated as a penis or cirrus, or project permanently beyond the surface of the body. The terminal portion of the oviduct is often widened so that two portions may be distinguished, the uterus, which harbors the eggs during their development, and the vagina, which serves for copulation. In addition there may occur in both sexes other accessory glands of the most diverse character. Oviduct and va
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